This invention relates to computer buses, and more particularly to digital communications buses that rely on reflected-wave switching for their operation.
Modem communications systems typically utilize some type of electronic bus system to provide data communications between various computer components. A bus system is characterized in terms of a number of different properties, including electrical properties, timing properties, and data communication protocols.
Physically, a computer bus comprises a set of signal lines that extend in parallel between multiple peripheral connectors or xe2x80x9cslots.xe2x80x9d Peripheral devices, sometimes referred to as xe2x80x9cexpansion cards,xe2x80x9d can be plugged into the peripheral connectors. Examples of such peripheral devices include memory devices, mass storage interface devices, sound cards, graphics cards, other I/O cards, etc.
In many cases, the bus is part of a so-called xe2x80x9cmotherboard,xe2x80x9d which incorporates a microprocessor and other components for controlling operations on the bus. In other cases, the bus is implemented as a passive xe2x80x9cbackplane,xe2x80x9d and controlling components are located on a peripheral device plugged into one of the peripheral connectors. The individual signal lines of the bus system are driven by components on the motherboard and/or by similar components on different peripheral devices.
High-speed computer bus systems typically utilize either incident-wave switching or reflected-wave switching. Signal line drivers are designed in accordance with the type of switching being utilized.
To understand the difference between incident-wave switching and reflected-wave switching, consider the case where a signal line or trace is fed by a driver and is attached to a number of device inputs distributed along the signal line. At high data speeds, each signal line acts as a transmission line, and the electrical characteristics of the transmission line must be considered when evaluating signal characteristics.
A transmission line presents impedance to any driver attempting to drive a voltage change onto a voltage line, and also imposes a time delay in the transmission of the voltage change along the line. Thus, when a signal line driver changes its output from one logic level to another, a wave propagates down the signal line, past the various peripheral devices coupled to the bus. As the wavefront propagates down the line, each device it passes detects the new logic level. Thus, each device detects the logic change at the first incidence of the signal along the signal linexe2x80x94at the incident wave. The amount of time it takes to switch all of the inputs along the line to a given value is the time it takes the signal to propagate the length of the line.
Another effect of a transmission line is that it produces reflections at points of high impedance. In the case of an unterminated signal trace, both ends of the trace present very high impedances. Since a signal cannot proceed beyond these points, it turns around and is reflected back down the bus. During the return passage of the wavefront, this effectively doubles the voltage change seen on the trace at each device""s input and at the driver that originated the wavefront. To prevent this, incident-wave buses typically use termination resistors on the ends of individual signal lines. The termination resistors dissipate the signal when it reaches the end of the signal line, and thereby prevent reflections. A termination resistor typically has a value equal to the transmission line impedance of the signal line itself. In some cases, resistive-capacitive termination circuits are used to terminate signal lines in incident-wave buses.
Although incident-wave switching results in the quickest possible switching of device inputs along a signal line, it has some negative aspects. One of the most significant disadvantages of incident-wave switching is that it requires relatively large drive currents. In addition, incident-wave switching is difficult to decouple, causes spikes on internal bond wires, increases EMI, and causes crosstalk with various components.
A reflected-wave bus reduces these problems by omitting termination resistors and using wavefront reflections to an advantage. A carefully selected, relatively weak output driver is used to drive the signal line halfway to the desired logic voltage. As the resulting wavefront passes each device input along the signal line, the voltage change is insufficient to register as a logic change. When the wavefront arrives at the unterminated end of the bus, however, it is reflected back and doubled. Upon passing each device input again during the wavefront""s return trip down the signal line, the new logic level voltage is registered at each device. The wavefront is eventually absorbed by the impedance of the signal line and of any peripherals coupled to the line, and the driver eventually increases its voltage to the steady state voltage of the new logic level. This cuts driver size and surge current in half.
The PCI (Peripheral Connection Interface) bus is one popular example of a computer bus that utilizes reflected-wave switching.
One problem with reflected-wave switching is that various critical timing parameters and characteristics can change depending on the length of the bus and on different load conditions. As an example, reflections in a non-terminated bus can continue beyond the first reflected wavexe2x80x94an incident wave can be reflected numerous times, back and forth between the ends of the bus. Driver and input device impedances can be designed so that this xe2x80x9cringingxe2x80x9d dies out quickly enough to avoid problems. However, different bus conditions (such as the number of load devices) can upset the balance obtained through the specification of bus impedances.
The PCI specification addresses this problem by strictly limiting bus length and the number of load devices. In this way, the bus design can be optimized for a limited range of conditions. In addition, the PCI specification recommends clipping diodes at each end of each signal line and at the signal line driver itself. The clipping diodes limit bus voltages to maximum upper levels and minimum lower levels, thereby limiting the intensity of reflected voltage waves. However, this method can lead to undesirable high-frequency noise.
In accordance with the invention, wave filters are used in a reflected-wave bus to partially dampen reflections. The wave filters are placed at each end of each signal line, and consist of low-pass RC filters. The low-pass RC filters have time constants such that an initial reflected wave is largely unaffected by a filter, but subsequent reflections are significantly damped. This allows reflected waves to be utilized for reflected-wave switching, while preventing continued ringing which might otherwise disrupt proper switching on the bus. Because of such RC filters, it is possible to design reflected-wave buses having longer lengths and higher loading than would otherwise be possible.
Although dampening waves in reflected-wave systems is contrary to accepted practice, the inventor has found it to be an advantage in certain cases in which strict physical compliance with bus specifications cannot be met.